This invention relates to a method for the treatment of wood to prevent decay, and more particularly, it is pertinent to a one-step treatment of wood with an aqueous solution of lignin materials and certain ammonia complexes of metals.
In the United States, as in many other countries possessed of abundant timber resources, wood has always been an important construction material. Its good availability, low heat conductivity, and sound-deadening qualities, have made it the outstanding building material since early human history. However, two major destructive forces of wood are fire and decay. While fire is often caused by human factors, decay is due :o biological attacks by microorganisms (such as fungi and bacteria), marine borers and insects. Much of the decay of wood in service is inevitable. It is particularly severe as a result of the activities of low forms of plants known as wood-destroying fungi. This type of fungi often causes extensive damage and financial loss to buildings in the temperate zone.
The conditions necessary for the development of decay-producing fungi in wood are: (1) a supply of suitable wood, (2) a sufficient amount of moisture, (3) at least a small amount of air, and (4) a favorable temperature. A deficiency in any of these requirements will inhibit the growth of a fungus, and reduce the decay of wood. Under normal service conditions of wood, the deficiency is difficult to maintain. So, a more common method for preserving wood against the fungal attacks is to impregnate it with certain wood preservatives. There are generally two types of wood preservatives. Oil-borne preservatives include creosote, pentachlorophenol, copper naphthenate, zinc naphthenate, copper-8-quinolinolate and others. Important water-borne preservatives are arsenic salts, boric acid, chromium salts, chromated zinc chloride, copper sulfate, mercuric chloride, sodium pentachlorophenate, zinc sulfate, chromated copper arsenate, etc.
According to a marketing study (Forest Industries, November 1985, pp. 36-38), chromated copper arsenate (or CCA) is the current most widely used lumber preservative in the U.S. About 95% of treated lumber (a total of 4 billion board feet in 1984) is treated with CCA, a water-borne preservative composed of metallic salts. CCA gives the lumber the best available protection against decay and insects. Southern yellow pine from southern lumber mills represents almost three-quarters of treated lumber. The rest is treated yellow pine lumber in the Northeast and treated pine in the Midwest. Wood preservatives such as those described above have been applied to the wood as solutions, emulsions, pastes or dispersions in liquid hydrocarbons and/or aqueous systems. In many applications, aqueous systems are preferred over liquid hydrocarbons because of the odors, flammability and often toxic nature of the hydrocarbon products. In contrast, water-borne preservatives tend to be less odorous, non-flammable, and give a clear appearance to the treated wood. Yet, aqueous preservative compositions often contain chromium and arsenate that are considered to constitute a significant risk to human health and the environment. For these and other reasons, a need exists for low-cost, non-hazardous, water-borne preservatives that contain no toxic chemical ingredients.
A common approach to achieving the object in the prior art involve fixation of certain metals such as zinc, copper, mercury, antimony, lead and so on in the wood with carboxylic organic compounds. For example, U.S. Pat. No. 4,783,221 (Nov. 8, 1988) teaches the use of a preservative compound comprising a metal salt of organic carboxylic acids of six to about 30 carbon atoms and an isothiazolone compound in preserving wood. Swedish Patent No. SE 430865 B (Dec. 19,1983) reveals a preservative composition containing copper or zinc salt of acetic and propionic acids. Australian Patent AU 519416 B2 (Nov. 12, 1981) discloses a mixture of copper oxide, decanoic acid and ethanolamine. Canadian Patent No. CA 978474 (Nov. 25, 1975) claims the use of zinc or copper salt of fatty acid as wood preservatives.
Lignin is a natural organic polymer that has recently been demonstrated in the prior art to be a suitable material for fixation of metals in wood. Thus, in U.S. Pat. No. 4,752,509 (Jun. 21, 1988), a method was disclosed for preventing the attack of wood by fungi and bacteria by the use of a modified alkali lignin. The method comprises the application of an aqueous solution of the alkali lignin modified into a water-soluble form, characterized in that the impregnation of wood takes place in two stages, these being an initial stage in which the aqueous solution containing the lignin, still in its water-soluble form and with a pH not exceeding 10, is applied to the timber (wood) in order to be absorbed by it, and a second stage in which the lignin is fixed being transformed into an essentially water-insoluble form in order to be retained in the timber, said second stage involving the application of a weakly acidic aqueous solution containing a metal salt or a combination of metal salts, said metal being zinc, copper or aluminum. The most common alkali lignin is the kraft lignin (or sulfate lignin) produced in the pulping of wood by the kraft process. The other alkali lignin is the lignin produced in the soda pulping process (soda lignin, currently not available commercially). Alkali lignin is generally water-insoluble and may be rendered water-soluble by certain carboxylation and sulfonation reactions as are practiced in U.S. Pat. No. 4,752,509. The nature of alkali lignin is such that in a mildly carboxylated and/or sulfonated form, it is water-soluble at neutral or slightly alkaline pH, but becomes insolubilized at acidic pH. The so-called water-soluble modified alkali lignin also tends to be "salted-out" by metal salts. This is the basic principle of fixation of lignin (with weakly acidic solution of metal salts) taught by the prior art. The two-stage method for impregnation of wood revealed in U.S. Pat. No. 4,752,509 suffers from several disadvantages which include, but not limited to, (1) costly drying of timber is required between the two stages, (2) the two-stage process is time-consuming since it involves loading of the same timber twice into the treatment vessel, and (3) the method is difficult and impractical to operate by the wood preserving industry due to equipment constraints.
Besides alkali lignin, the other major type of lignin produced by the paper industry is sulfite lignin (or lignosulfonate) from pulping of wood by the sulfite process. Lignosulfonate is water-soluble at all pH's, including acidic, neutral and alkaline pH's. In general, it cannot be insolubilized by addition of mineral acids or metal salts. Therefore, lignosulfonate as derived normally from the sulfite pulping liquors cannot be "fixed" into the wood by aforementioned two-step method. Lignosulfonate absorbed into the wood that is subsequently treated with acidic aqueous solution of metal salts such as copper and zinc salts, is easily leached out of the wood by water and it loses its effectiveness as a wood preservative.
Between the two commercial lignins available in the marketplace, lignosulfonate is produced in a quantity (1,340 million kilograms per annum) at least 30 times greater than that of kraft lignin. The unproportionally greater quantity makes it attractive and desirable to produce a wood preservative composition based on sulfite lignin. Thus, in U.S. Pat. No. 4,988,576 (Jan. 29, 1991) is revealed a wood preservative composition that consists of a lignosulfonate copolymer and a metal salt (plus optionally methylol phenol), and that can be applied to the wood by the standard one-step pressure/vacuum process commonly practiced in the industry. The distinction between the two prior art disclosures on lignin-based wood preservatives is clear in that while the kraft modified lignin is "fixed" (or rendered insoluble) in the wood by acid and metal salts (a salting-out effect), the lignosulfonate copolymer is retained in the wood by in situ condensation by itself or with methylol phenol during the drying process. Both of the lignin prior art disclosures have clearly demonstrated the efficacy of lignin as a fixation agent for metal salts in the wood and as a component in wood preservative compositions. Nevertheless, neither of the disclosures is free from deficiencies. The two-stage method of U.S. Pat. No. 4,752,509 is not readily adaptable to the industry, and the preservative of U.S. Pat. No. 4,988,576 has to resort to the use of certain expensive lignosulfonate copolymers. In addition, both the prior art methods for impregnation of wood achieve only limited penetration of wood preservative in the wood. Therefore, it will be advantageous to have a wood treatment method that utilizes a preservative composition solely based on lignin, and that is applicable to the wood by a one-step method.
It is thus an object of this invention to provide a one-step method for treatment of wood to achieve its protection against decay.
It is a further object of the invention to provide a wood treating method that utilizes low-cost lignin or modified lignin.
It is another object of the invention to provide a method for treating wood that utilizes both lignosulfonate and alkali lignin.
It is an additional object of the invention to provide a method for impregnation of wood with lignin-based preservatives, whereby good penetration of said preservatives into the wood is attained.
It is another important object of this invention to provide a method for treatment of wood, whereby desirable fixation of lignin-metal salts in the wood is made possible.